How to model effectively using exact measurements?
The absolute first step is to set the real-world units in the scene tab. This will make Blender's arbitrary units relate to something you can actually measure, like feet or meters.
Lines/Edges that Follow an Axis
You can click on a vertex, press E to extrude, press X, Y, or Z to choose the direction, then type in 1 and hit Enter. This will produce a line of length 1 (scene units) in that direction.
Lines/Edges at Arbitrary Angles
If your line needs to be at an angle, and you know how long it needs to be, then you could:

Create an edge with that length. Position the 3D cursor on one of
the vertices of the edge.

Set the pivot centre to be on the 3D cursor. (important)

Rotate the edge: (select the edge, hit R, hit (X,Y, or Z for axis) and type in the angle to rotate on that axis) , then Enter to finalize

Rinse and repeat until all axis of that edge are dealt with.


Extruding Edges/Faces
Edges and Faces can also be extruded and knowing how to do it can save you a lot of time:

select an edge or face, hit E and choose an axis using X,Y, or Z and type in the distance, then Enter.
Faces will start to extrude along their normal if no X,Y, or Z direction is given.

Directly entering coordinate values (global / local)
Get familiar with entering exact coordinates into this box. (found on the right hand panel, the N-panel). In Global mode when you change the position of a selected vertex using those fields then the coordinate will be relative to the world origin (0,0,0). When in Local mode the coordinate will be relative to the origin of the Object.